ActionScript: The Definitive Guide

Dynamic Movie Clip Event Handlers

Early in this chapter, we learned about two kinds of events in Flash—those that are attached to movie clips and buttons and those that are attached to other data objects such as XML and XMLSocket. To create event handlers for data objects, we assign the handler function name as a property of the object. Recall the syntax to add a function dynamically:

myXMLDoc.onLoad = function ( ) { trace("all done loading!"); };

Dynamic function assignment lets us change the behavior of the handler during movie playback. All we have to do is reassign the handler property:

myXMLDoc.onLoad = function ( ) { gotoAndPlay("displayData"); };

Or we can even disable the handler altogether:

myXMLDoc.onLoad = function ( ) { return; };

Unfortunately, handlers of movie clip and button events are not nearly so flexible; they cannot be changed or removed during movie playback. Furthermore, movie clip event handlers cannot be attached to the main movie timeline of any movie! It’s impossible to directly create an event handler for a movie’s _root clip.

In order to work around these limitations, we can—in the case of the enterFrame and the user-input events—use empty movie clips to simulate dynamic event-handler removal and alteration. Empty movie clips even let us simulate _root-level events. We’ve already seen the technique in Chapter 8, where we learned how to create an event loop as follows:

1. Create an empty movie clip named process.

2. Place another empty clip called eventClip ...